The present invention refers to an improved apparatus for the production of containers made of thermoplastic material, in particular polyethylene terephthalate (PET) and polypropylene (PP), intended for use in applications employing them being filled with liquids at elevated temperatures and/or containing CO2 (carbon dioxide) gas, provided with devices that enable the productivity of moulds during the blow moulding phase to be increased.
In particular, the present invention is particularly advantageous when the described devices are associated with a preform manufacturing apparatus included in a so-called single-stage plant, but can be advantageously used also in conjunction with apparatuses that are supplied with previously manufactured preforms and are solely intended to carry out a final blow moulding phase (ie. two-stage plants).
In a two-phase process, a previously produced preform or parison, which is in a substantially amorphous state, is heated up again to its preferred molecular orientation temperature, at which it is then blow-moulded into the desired shape. As used in this context, the term xe2x80x9ctwo-stage processxe2x80x9d, or xe2x80x9cdouble-stage processxe2x80x9d, shall be understood to cover any process that produces a preform or parison which must then be heated up from ambient temperature to the related blow-moulding temperature.
In contrast therewith, single-stage processes are so defined in that they are capable of forming the so-called preform, or parison, and transferring the preform from the injection mould or extrusion die (upon it having been allowed to cool down to some appropriate temperature) to a conditioning station, where it is allowed to level evenly at a temperature of preferred molecular orientation. The preform or parison is then transferred to a blow-moulding mould, in which it is finally moulded into its desired form. According to a prior-art technique, the cavities in which the preforms are injection moulded and the preform transferring and conditioning devices are arranged in an on-line configuration along with the blow-moulding moulds, so as to ensure an easier, more convenient handling of the preforms, the containers and the various members therewith associated.
According to such a construction principle, both preforms and finished containers are processed in successive groups, with the same processing and/or transferring operations occurring at the same time.
In particular, the blow moulding tools, the the moulds, are particularly critical in this connection, since with the increasing blow moulding pressure and the increase in the number of cavities contained in each pair of blow moulding platens, more rapid and powerful pumping stations are required. Furthermore, the increased total pressure produced by the bottles during blow moulding must be opposed by a corresponding greater mould clamping pressure.
Such a greater pressure, which can be estimated to amount to a clamping force exerted on the moulds in excess of 100,000 kgf, would require all mechanical and pneumatic organs intended to produce and withstand such a pressure to be sized accordingly, which generally means to a very burdensome extent.
Such a huge and, what""s more, pulsating pressure, however, has a negative effect also on the resistance of the moulds themselves, which are not only exposed to a greater pressure, but have, at the same time, to be capable of withstanding such pressure over a much greater period, owing to the greater number of blow moulding gates, and this of course causes the rigidity thereof to become a critical factor. It also makes it easier for the moulds to warp outwardly, thereby affecting the bottle blow-moulding results in an easily understandable manner.
Furthermore, when use is made of blow moulding moulds provided with a large number of cavities, the time needed for all preforms to be transferred into the respective cavities increases in a proportional manner with a corresponding increase in the cycle time and a resulting reduction in the overall productivity of the plant.
In order to do away with such drawbacks, a largely known solution lies in the replacement of blow moulding moulds provided with a plurality of on-line cavities with a smaller number of individual moulds, ie. comprising a single respective blow moulding cavity, which are arranged along the periphery of a rotating carousel-like structure, such as for instance illustrated in WO 95/05933, WO 89/01400, U.S. Pat. Nos. 4,850,850 and 4,313,720.
In particular, WO 89/01400 teaches that the blow moulding half-moulds are adapted to open and close in a book-like manner about respective axes of rotation that are orthogonal to the plane on which the preforms and the finished containers move.
Such a solution, although effective in solving the afore cited problems, does not however go without some drawbacks that are summarized below:
a) the need arises for two half-moulds to be handled and driven, and this, of course, adds to the complexity of the structures and causes costs to rise;
b) the lateral rotary movement of both the half-moulds requires, of course, the availability of adequate lateral space, and this, of course, puts a penalty on the compactness of the plant;
c) the handling means provided must be able to introduce the preforms from the front zone of the half-moulds, as well as to again remove the respective finished containers from the same front zone, and this of course understandably adds significantly to the complexity of both construction and operation of the plant.
However, during operating tests carried out on some types of plants having the characteristics as recited in the appended claims, and made with the use of known techniques of more obvious and immediate choice, such as the use of a direct pneumatic control or electromagnetic actuators or means like a lever joint generally known as a toggle joint in the art, it has been observed that a number of problems tend to a arise in connection with the opening and closing, or clamping, of the moving half-mould, ie.:
the final closing and opening movement of the moving half-mould is an abrupt, not adequately slowed-down one, so that it may give rise to blows and clattering;
owing to the rapid wear-down of the mechanical organs and parts concerned (it should be noted that these must be able to operate on a contmuous-duty basis over very long periods of time), the duration of the machine life is significantly affected, ie. reduced, thereby giving rise to immediate problems of planned maintenance and/or repair work;
the noise generated by the entire system controlling and actuating the movement of the moving half-mould is in general quite high and tends to further increase owing the the above cited wear-down effect;
since the moving half-mould must perform an alternating opening and closing movement at a relatively high rate, vibrations are generated which tend to be transferred to the entire structure of the plant, and this has obviously a negative impact on the duration and the operation of the machine organs involved; furthermore, the final opening and closing phase of the moulds is an extremely short and therefore strongly accelerated phase, and this of course contributes to an increased level of the so induced vibrations; and
for such accelerations to be obtained, over-sized movement control and driving organs must be used, but this fact not only has a negative effect on the level of the vibrations, but also, due to the greater extent of wear-down caused, to the ability of ensuring the desired precision standards over adequately long periods of time.
Based on these considerations, it therefore is a main purpose of the present invention to provide a blow-moulding apparatus of a plant for producing hollow bodies allowing for the productivity thereof to be increased through an accelerated handling of both preforms and finished containers, without incurring any of the afore mentioned drawbacks, which is capable of being easily implemented with the aid of readily available techniques and means and therefore is reasonably low-cost, reliable and preferably capable of being integrated with a preform production stage arranged upstream.